O’Brien, a longtime Massachusetts political hand, will rejoin his former boss, Secretary of State John Kerry, working in Washington on a global partnerships program launched under former secretary Hillary Clinton.

A former longtime top aide to Boston Mayor Thomas M. Menino, O’Brien later worked as Kerry’s in-state chief of staff. At Foggy Bottom, he will be the State Department’s Special Representative for Global Partnerships Initiatives, which leverages public-private relationships to help provide assistance like clean water and cookstoves around the world.

“I’m absolutely excited by this opportunity the secretary offered,” O’Brien told the Globe. “And I’m looking forward to working with him once again.”

After Kerry took the top diplomatic post, O’Brien remained in the state director role under interim Senator William "Mo” Cowan. Longtime Bay State political observers credit O’Brien with helping Kerry improve local relationships after the former senator’s 2004 presidential bid failed.

O’Brien and Kerry are personally close. When O’Brien’s father, Edward L. O’Brien, died in 2011, Kerry eulogized him on the Senate floor.

A Marshfield native, and West Roxbury resident, O’Brien also worked for state treasurer Shannon O’Brien and as a deputy chancellor at the University of Massachusetts at Boston. His brother-in-law is Hyde Park city councilor Rob Consalvo, a mayoral candidate.

O’Brien is expected to split time between Boston and the capital. His wife, Michelle Consalvo, and their 10-year-old twins plan to remain in Massachusetts.

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